Glitchy-looking transparent alpha channel PNG when imported as "Image as Plane" in Cycles
When you click import images as planes and the file browser opens, in the lower right, under 'import images as planes', you need to check the  use alpha checkbox, or you will get bizarre alpha junk.  Why this isn't a default I have no idea. 



As an alternative on the UV/image editor press N to bring up the options window and enable "Use Alpha"